In the recovery of oil from subterranean oil-containing reservoirs, various flooding techniques have been employed, the most common being waterflooding. However, such techniques are rapidly becoming impractical from the perspective of cost and efficiency. Many oil wells will produce a gross product effluent comprising 80-98 percent by volume of water, and hence, only 2-20 percent by volume of oil. As will be easily recognized, most of the pumping energy is expended in lifting water from the well, requiring the production effluent to undergo expensive separation procedures to recover water-free hydrocarbons. In addition, the residual foul water constitutes a troublesome and expensive disposal problem.
The inordinate amount of water present in these subterranean reservoirs can occur from the infiltration of naturally occurring subterranean water or from injected water. The excessively high water content effluent is primarily due to the fact that different strata or zones in the reservoir often vary in permeability, so that displacing fluids, or "drive" fluids, enter the high permeable or "thief" zones in preference to zones of lower permeability where significant quantities of hydrocarbons may be left.
Thus, in order to maximize the hydrocarbon content and minimize the water content of the production effluent resulting from waterflooding, and to maximize the volumetric sweep efficiency of the driving fluid, the desirability of designing a viscous slug capable of sealing off the more permeable or thief zones so that the floodwater would be diverted to the underswept, tighter regions of the reservoir becomes evident. The more preferred viscous slugs of this nature, typically labeled "profile control agents", have included oil/water emulsions, gels and polymers, with polymers being the most extensively applied in recent years.
Proposals have been made for the use of inorganic polymers, especially inorganic silicates, as permeability control agents. For example, U.S. Pat. Nos. 4,009,755 and 4,069,869 disclose the use of inorganic silicates for this purpose. In the permeability control method described in these patents, an organic polymeric permeability control agent such as a cross-linked polyacrylamide or polysaccharide is first injected into the reservoir, followed by an aqueous solution of an alkaline metal silicate and a material that reacts with the silicate to form a silicate gel which plugs the high permeability regions in the formation. An alkaline metal silicate is typically used as the source of silica and the gelling agent is usually an acid or acid forming compound such as a water soluble ammonium salt, a lower aldehyde, an aluminum salt or an alkaline metal aluminate.
The problem, however, with many inorganic silicates is that their solutions are often quite viscous and stable only under alkaline conditions. As soon as conditions become acidic, a silicate gel is formed. Although this is the desired reaction for plugging the formation, it may occur prematurely, i.e., before the solution has had an adequate opportunity to enter the high permeability regions of the formation, cutting off the possibilities for further injection of plugging material.
Other attempts have been made to achieve profile control. One such attempt is described in U.S. Pat. No. 4,498,539 to Bruning, which discloses delayed gelable compositions for injection of a water thickening amount of a polymer capable of gelling in the presence of a cross-linking agent so that after the composition has penetrated into an underground formation and positioned itself preferentially in the highly permeable strata, the delayed gelation is triggered by in situ hydrolysis of an ester which reduces the pH of the composition to the gelable range thereby producing in-depth plugging of the strata with the gelled polymer.
U.S. Pat. No. 4,417,623 to Anthony describes a method for consolidating sand with organic silicate wherein unconsolidated sand-like material in a subsurface formation adjacent a borehole of a water, oil, or gas well is consolidated by treating the formation first with a solution of alcohol and organic silicate and then with water. The water causes the organic silicate to hydrolyze and polymerize into a coating-like binding agent. The water also flushes excess organic silicate-alcohol solution from the more permeable portions of the formation adjacent the borehole, thereby maintaining the formation's permeability.
U.S. Pat. No. 4,081,029 to Holm discloses a method for enhancing oil recovery from subterranean reservoirs which includes injection of a relatively large slug of a dilute aqueous alkali metal silicate solution followed optionally by an aqueous drive fluid, again followed by a small slug of a dilute aqueous solution of an agent that reacts with the alkali metal silicate to form a gelatinous precipitate.
U.S. Pat. No. 4,011,910 to Rhudy et al. discloses mobility control in secondary type oil recovery through injection of two aqueous polymer solutions. The polymer of the first solution has an average molecular weight of at least 10 million while the second polymer solution contains a polymer which does not substantially change rock permeability, but imparts a viscosity increase to the solution. An example of the polymers used in the first solution is high molecular weight polyacrylamide while the second solution can contain a biopolymer such as a polysaccharide.
U.S. Pat. No. 3,981,363 to Gall discloses a method for obtaining good residual resistance factor at relatively low ratios of cross-linking agents to polymer for plugging fractured porous media by injecting into the formation a first aqueous polymer solution followed by injecting a cross-linking agent capable of gelling the polymer solution and thereafter injecting a second aqueous polymer solution that is capable of being gelled by the cross-linking agent. In each of the aqueous polymer solutions the polymer is already partially cross-linked before the polymer solution is injected into the formation. Furthermore, each of the injections of polymer solution are whole-slug injections.
U.S. Pat. No. 3,833,061 to Gall discloses a method for selectively reducing the permeability of an oil-wet subterranean formation by passing an oxidizing agent through and in contact with the formation for oxidizing and removing hydrocarbon from the surfaces of the formation and thereafter contacting the treated formation surfaces with a cross-linked polymer for selectively reducing the permeability of the formation to brine while maintaining the permeability of the formation to hydrocarbon fluids relatively unchanged.
U.S. Pat. No. 3,762,476 to Gall discloses a method for correcting water permeability of a well bore-penetrated subterranean formation by injecting a first aqueous polymer solution, a complexing ionic solution, a brine slug, a second aqueous polymer solution, terminating the injection of the second aqueous solution, and recovering the hydrocarbon fluids from the subterranean formation.
U.S. Pat. No. 3,757,863 to Clampitt et al. discloses a method for reducing the quantity of water recovered from a subterranean formation by treating the formation with an acid, a neutralizing brine, and at least one slug of thickened aqueous solutions.
While these patents disclose methods for achieving profile control to a limited extent, each of them suffer disadvantages which detract from the efficiency and quality of secondary oil recovery.
Accordingly, it is an object of the present invention to provide a method of attaining improved profile control of subterranean oil-bearing strata or stratified reservoirs.
It is another object of the present invention to provide such a method which maximizes the oil content and minimizes the water content of production effluent resulting from waterflooding oil-recovery operations of subterranean formations.
It is another object of the invention to maximize invasion of hydrocarbon containing regions of the formation by the driving fluid.
It is a further object of the present invention to make practical, waterflooding oil-recovery operations, from the perspective of cost and efficiency.